4.2 Quantitative Ultrasound (QUS) Principles & Role
Key Takeaways
Heel QUS uses non-ionizing sound waves. Speed of sound is expressed in m/s and broadband ultrasound attenuation is the slope in dB/MHz.
The heel is the validated clinical QUS site. Use a validated device and apply risk thresholds with clinical risk factors.
Heel QUS can inform fracture risk assessment but its T-score does not establish WHO osteoporosis. Central DXA is preferred when feasible.
QUS is unsuitable for monitoring the skeletal effects of osteoporosis treatment.
Identify the acoustic measurement
Quantitative ultrasound (QUS) uses mechanical waves, so the examination uses no ionizing radiation. In a transmission heel sonometer, a transducer sends a pulse through the heel and a second transducer detects it. The instrument uses its specified coupling system, such as a water bath or gel pads. Align the heel and follow the manufacturer's instructions for reference checks, positioning and coupling. A poor acoustic path can affect the result.
| Quantity | What the instrument reports |
|---|---|
| Speed of sound (SOS), in m/s | A speed estimate derived from the propagation path and transit time. |
| Broadband ultrasound attenuation (BUA), in dB/MHz | The slope of attenuation versus frequency across the instrument's analysis band. |
| Composite acoustic index | A device-specific combination of SOS and BUA. Examples include the Achilles InSight stiffness index and the Sahara quantitative ultrasound index. |
For a fixed known path length and travel time , the simple speed calculation is . In a heel measurement, the device accounts for its own geometry and reference path. Tissue density and elastic properties influence acoustic speed, but a heel SOS reading cannot isolate either one. Acoustic absorption and scattering contribute to attenuation. BUA can be associated with bone composition and structure, but it does not directly measure trabecular connectivity.
If attenuation increases from 12 dB at 0.3 MHz to 28 dB at 0.5 MHz, the illustrative slope is dB/MHz. A sonometer estimates its slope over a defined frequency band. This example teaches the units and does not provide a clinical threshold.
As a device-specific example, the published GE Achilles InSight protocol calculates a stiffness index of using the expected input units. BUA of 70 dB/MHz and SOS of 1,550 m/s give 60.9. The index combines acoustic measures. It does not directly measure an elastic modulus. The Sahara device can report an estimate of heel BMD. Even when such an estimate uses g/cm², it is not interchangeable with a central DXA BMD or an appropriate femoral neck input for FRAX.
Use the result for the supported clinical question
The International Society for Clinical Densitometry (ISCD) identifies the heel as the only validated QUS site for clinical osteoporosis management. Its adult positions recognize fracture prediction by validated heel devices for hip, vertebral and overall fracture risk in postmenopausal women and for hip and nonvertebral fractures in men over age 65. An acoustic result and central DXA can disagree because the methods use different measurements. Review quality and context before calling a discrepancy an error. A patient can have low heel QUS and a non-osteoporotic hip DXA T-score. The clinician can use the valid DXA result for densitometric classification while incorporating the heel result into the clinical risk assessment. The results should retain their separate device references.
Use clinical risk factors and thresholds validated for the specific device and population. A sufficiently low fracture probability can support deferring further assessment in some patients. Central DXA is preferred for treatment decisions when feasible. If it cannot be performed, ISCD allows treatment consideration when a validated heel QUS assessment and clinical risk factors together indicate sufficiently high fracture probability. A heel QUS T-score cannot establish the WHO densitometric diagnosis. Do not use serial QUS to monitor the skeletal effects of osteoporosis treatment.
Worked referral decision
A 67-year-old postmenopausal woman has a heel QUS result in the indeterminate range under a locally validated device-specific referral pathway. She has no known fractures and central DXA is available. Obtain the indicated central DXA and review clinical risk. An indeterminate acoustic result cannot establish normal density or osteoporosis. Repeating it after three months would not establish a clinically useful rate of bone loss.
Now consider a patient with a high-risk heel QUS result who cannot have central DXA. Confirm the correct device-specific threshold and review fracture history and clinical risk before making a treatment decision. Do not convert the acoustic score to a WHO diagnosis. In either case, report the device, measured site, acoustic parameter or index and relevant limitations so that another reader can tell which evidence applies.
ISCD Adult Official Positions · Original BUA measurements · FDA Sahara documentation · Achilles InSight study protocol · OsteoLaus fracture cohort
Attenuation rises from 12 dB at 0.3 MHz to 28 dB at 0.5 MHz. What does this imply for the illustrative broadband ultrasound attenuation slope?
8 dB/MHz, a measure of hip mineral mass.
16 dB/MHz, a count of connected trabeculae.
80 dB/MHz, describing frequency-dependent attenuation.
140 dB/MHz, a measurement of cortical elastic modulus.
Under ISCD adult positions, how may a validated heel QUS result be used when central DXA cannot be performed?
It can contribute to a treatment decision if device-specific thresholds and clinical factors together indicate sufficiently high fracture probability.
A QUS T-score at or below −2.5 by itself establishes the WHO densitometric diagnosis.
A repeat heel QUS one year later can establish an antiresorptive response.
A heel BMD estimate can replace femoral neck BMD in standard FRAX.
A 67-year-old postmenopausal woman has an indeterminate heel QUS result under a validated referral pathway. Central DXA is available. What is the most appropriate next assessment?
Start medication solely from the indeterminate result.
Obtain central DXA and review the patient’s clinical risk.
Repeat QUS after three months to measure the rate of bone loss.
Discharge the patient from risk assessment because the result proves normal BMD.
Sections you finish are checked off in the contents.